As a power source of a mobile device, or the like, a lithium ion secondary battery is mainly used. The function of the mobile device or the like is diversified, resulting in increasing in power consumption thereof. Therefore, a lithium ion secondary battery is required to have an increased battery capacity and, simultaneously, to have an enhanced charge/discharge cycle characteristic. Further, there is an increasing demand for a secondary battery with a high output and a large capacity for electric tools such as an electric drill and a hybrid automobile. In this field, conventionally, a lead secondary battery, a nickel-cadmium secondary battery, and a nickel-hydrogen secondary battery are mainly used. A small and light lithium ion secondary battery with high energy density is highly expected, and there is a demand for a lithium ion secondary battery excellent in large current load characteristics.
In particular, in applications for automobiles, such as battery electric vehicles (BEV) and hybrid electric vehicles (HEV), a long-term cycle characteristic over 10 years and a large current load characteristic for driving a high-power motor are mainly required, and a high volume energy density is also required for extending a driving range (distance), which are severe as compared to mobile applications.
In the lithium ion secondary battery, generally, a lithium salt, such as lithium cobaltate, is used as a positive electrode active material, and a carbonaceous material, such as graphite, is used as a negative electrode active material.
Graphite is classified into natural graphite and artificial graphite.
Among those, natural graphite is available at a low cost. However, as natural graphite has a scale shape, if natural graphite is formed into a paste together with a binder and applied to a collector, natural graphite is aligned in one direction. When an electrode made of such a material is charged, the electrode expands only in one direction, which degrades the performance of the electrode. Natural graphite, which has been granulated and formed into a spherical shape, is proposed, however, the resulting spherical natural graphite is aligned because of being crushed by pressing in the course of electrode production. Further, the surface of the natural graphite is active, and hence a large amount of gas is generated during initial charging, which decreases an initial efficiency and degrades a cycle characteristic. In order to solve those problems, Japanese Patent publication No. 3534391 (U.S. Pat. No. 6,632,569, Patent Document 1), etc. propose a method involving coating artificial carbon on the surface of the natural graphite processed into a spherical shape.
With respect to artificial graphite, negative electrode materials using so-called hard carbon and amorphous carbon described in JP 07-320740 A (U.S. Pat. No. 5,587,255, Patent Document 2) are excellent in a characteristic with respect to a large current and also have a relatively satisfactory cycle characteristics.
Artificial graphite typified by a graphitized article of oil, coal pitch, coke and the like is available at a relatively low cost. Particularly, with respect to the artificial graphite made of non-needle-shaped coke, it is possible to make it have a relatively high energy density per volume by performing heat treatment at a high temperature to obtain a negative electrode material which satisfies cycle characteristics and high rate characteristics required for a large battery. It is assumed that these materials can deliver high performance due to a moderate disorder in crystals because the materials have a crystallinity not so high as natural graphite but higher than hard carbon and amorphous carbonaceous materials.